High speed machining center

ABSTRACT

A micro-machine powered by an electric motor and/or a gas turbine drive and capable of rotation speeds to 1,500,000 rpm is disclosed. The micro-machine may include a rotating spindle which is constructed, balanced and arranged to provide a tool holder function and may incorporate a portion of the drive system. The other portion of the drive system is incorporated in a machine housing. The housing locates and supports at least one foil thrust bearing and at least one foil journal bearing to support the spindle. The housing and the bearings may be split into at least two parts and reassembled and the housing incorporates a gas reservoir and gas distribution channels for forced gas cooling of the bearings. In one embodiment of a foil journal bearing, a plurality of overlapping top foil segments supported by individual bump foil segments consisting of generally uniformly-spaced ridges and flats are secured within a housing. The bearing may be split and the bump foil stiffness may be adjusted by modifying the geometry, number and/or arrangement of the bump foils. In one embodiment of a foil thrust bearing a plurality of top foil segments are supported by coextensive individual bump foil segments including ridges and flats and secured on a thrust plate. The bearing may be split. The bump foil may be divided into a series of tabs separated by slits, each of the tabs having a number of ridges separated by flats. The ridges and flats on adjacent tabs may be arranged to reduce bearing heating. Provision may be made for cooling of the bearing by forced radial gas inflow.

This application claims the benefit of U.S. Provisional Application61/278,385 filed Oct. 6, 2009.

TECHNICAL FIELD

The technical field relates to the development and application of highspeed machining and grinding machines, particularly those suitable forfabrication of microscopic features, and of high speed foil journal andthrust bearings suited for use in such a micro-machine.

BACKGROUND

It is well known in the machining arts that cutting tools perform bestwhen urged into contact with a workpiece at a specific speed or within aspecific range of speeds. Although the particulars of the speed rangemay vary with workpiece composition or workpiece attributes such ashardness or ductility, this behavior is generally observed. Inparticular it is observed in both metal and ceramic workpieces, for toolsteel, carbide, coated carbide and ceramic tools, and for cutting toolsof specified geometry such as milling cutters as well as for toolscomprising a bonded assemblage of a more or less randomly-orientedcutting edges such as diamond or ceramic grinding tools.

Many rotating cutting tools, such as mills, burrs and drills, mount thecutting edges at their periphery. Thus as the tool diameter is reducedto enable the creation of smaller features in the workpiece, commonlytermed micro-machining, the tool is required to rotate faster tomaintain the preferred peripheral cutting speed range since the linearvelocity is given by the product of the angular velocity and the toolradius.

For purpose of illustration only, a reasonable value for the preferredcutting speed of aluminum is about 75 meters per minute. Thus a rotarytool with a radius of about 500 micrometers (0.5 millimeter) should beoperated at a rotational speed of about 25000 revolutions per minute(rpm). Reducing the tool diameter to about 50 micrometers leads to atool rotational speed of 250,000 rpm and still further reduction toabout 25 micrometers would necessitate a tool rotational speed of about500,000 rpm for the cutting tool were to operate in the preferred range.

Thus micro-machines capable of micro-machining must, for robust cuttingperformance, operate at significantly higher rotational speeds thanconventional machine tools. More specifically, for machined features 100micrometers in width or less the micro-machine should be capable ofoperation at several hundreds of thousands of rpm which posessignificant challenges in the manufacture and operation of such devices.

As the size of the machined feature shrinks the need for high precisionin the micro-machine increases. For example control of tool runout tomicrometer levels is required placing stringent requirements ontool-micro-machine attachment systems and on machine spindle alignmentand runout among other issues. Since the machine spindle will besupported on bearings, many of the required micro-machine featurespromote a need for innovative bearing designs.

In turn the machine spindle and bearings must be assembled into asupport structure, or housing. It may therefore be important that thehousing, bearing and spindle design be consistent with assemblypractices which assure high precision in the assembled micro-machine.The assembly practices should be robust, that is, accepting of normalpart or component tolerances and assembler skill level, withoutsignificant prejudice to performance. The assembly practices should alsoenable disassembly and reassembly without significant prejudice toperformance.

SUMMARY

One embodiment may include a high speed micro-machine with a drivesystem for rotation of a spindle about a rotation axis and supported byat least one gas-cooled foil journal bearing and at least one gas cooledfoil thrust bearing. The bearings in turn may be supported by a housing,with a pressurized gas reservoir. The housing may be split along atleast one joint line into at least two parts. The housing may contain apressurized gas reservoir. The joint line may lie in a planesubstantially containing the axis of rotation or may lie in a planesubstantially perpendicular to the axis of rotation. Both types of jointlines may be present simultaneously.

The micro-machine may be powered by a gas turbine or an electric motoror both, and, if powered by an electric motor, may incorporate acompressor for generation of pressurized gas for cooling the bearings.The drive system may be partitioned with a portion located on thespindle and a portion located on the housing.

The spindle may have a hollow portion bounded by an endcap with inclinedthrough holes for capture and retention of machining debris. Extendingfrom the endcap there may be a solid cylinder with, on its end, a toolholder for acceptance and retention of a tool shank.

The foil bearings, both thrust and journal, may be constructed of anumber of top foil segments, supported by a like number of bump foilsegments and mounted to and supported by a housing or support. Thejournal bearings may have the form of a hollow cylinder with the topfoil and bump foil segments arranged around the interior surface; thethrust bearings may have the form of a disc-like thrust plate with bumpfoils and top foils mounted on one of the planar disc surfaces. The foilbearings, both thrust and journal, may be split and reassembled tofacilitate assembly of the micro-machine. The foil bearings may becooled by provision of pressurized gas flow directed along the channelsin the housing. The gas flow may be directed along the cylindrical axisof the journal bearings and radially inward, that is from the edge ofthe generally disc-like thrust plate toward its center, for the thrustbearings.

In one embodiment a foil journal bearing may include a plurality of topfoils overlying a like number of bump foils, each supported by theinterior circumference of a hollow, generally cylindrical housing havinga length. The widths of the top foils and bump foils may besubstantially equal to the length of the housing. The bump foils mayhave a plurality of ridges and flats oriented generally parallel to thecylinder axis. Each of the top and bump foils may be attached to thehousing. Attachment may be by engagement of mounting features on thefoils which extend across substantially their width with complementaryfeatures in the housing. The complementary features in the housing maybe uniformly distributed around the interior circumference of thehousing.

The mounting feature of a top foil may be located between the ends ofthe top foil to thereby divide the top foil into a leading segment and atrailing segment. The top foils may be arranged so that the leadingsegment of a first top foil overlies the trailing segment of an adjacenttop foil. Each bump foil may have a length generally equal to the lengthof the leading segment and secured at a single location to underlie theleading segment of one of the top foils.

In a second embodiment a foil journal bearing may include a top foiloverlying a bump foil each supported by the interior circumference of ahollow, generally cylindrical housing having a length. The widths of thetop foil and the bump foil may be substantially equal to the length ofthe housing. The bump foil may have a plurality of ridges and flatsoriented generally parallel to the cylinder axis. The top foil and thebump foil may each have a length substantially equal to the interiorcircumference of the housing, and a width substantially equal to thelength of the housing. Each of the top foil and bump foil may have amounting feature, extending substantially across its width, forengagement with a feature of complementary shape in the housing. Thebump foil may have a plurality of regions, each including groups ofgenerally uniformly-spaced ridges and flats, the regions being separatedby extended flat regions.

In a third embodiment a foil journal bearing may include a single foilsecured to and supported by a hollow, generally cylindrical housing witha cylinder axis, an interior circumference and a length. The width ofthe foil may be substantially equal to the length of the housing. Thelength of the foil may be substantially equal to twice the interiorcircumference of the housing and the foil may have, a mounting feature,extending across its width. The engagement feature may engage a featureof complementary shape in the housing. The foil may have two portions ofapproximately equal length where one portion of the foil may be a bumpfoil having a plurality of regions each having groups of generallyuniformly-spaced ridges and flats, the regions being separated byextended flat regions. And where the other portion of the foil may be agenerally flat top foil which may overlie the bump foil which mayoverlie the interior of the housing. The mounting feature may bepositioned at about the midlength of the foil, or, alternatively, at theend of the bump foil portion of the foil.

One embodiment may include a foil thrust bearing comprising a pluralityof generally planar top foils overlying a like number of coextensivebump foils, which may be supported by a generally disc-like thrust platewith a center and a circumference.

The top foils, and their associated bump foils may be positioned in theannular region formed between two circles, an inner circle and an outercircle, where each circle may be centered on the thrust plate center.The foils may be bounded by four edges; on two opposing edges, the edgesmay have the form of circular arcs whose radii correspond to the radiiof the inner and outer circle. The two other opposing edges are linearand may be portions of radial lines lying between the inner and outercircle. One of the linear ends of both the bump foil and thrust foil maybe free and not secured to the thrust plate. One of the linear edges ofthe top foil and one of the linear edges of the bump foil may be securedto the thrust plate. The foils may be welded to the thrust plate ormechanically secured, for example by means of a structure on the edge ofthe foil engaging a slot or other structure of complementary shape inthe thrust plate. The foils may be generally equally spaced around theannular region and separated by gaps between adjacent foils.

Each of the bump foils may be divided, into a series of circumferentialtabs by a number of circumferentially-oriented slots extending from thefree end of the foil part-way toward the secured end of the foil. Eachof the tabs may be corrugated to form a series of substantially parallelridges separated by flats, each of the ridges and flats being uniformlyand substantially equally spaced apart and each of the ridges and flatsbeing oriented generally parallel to the secured edge. Each of theridges may be characterized by a peak with a height, with each of theflats having a centerline oriented generally parallel to the ridges.

The thrust plate may have a plurality of openings which permit theradial inflow of cooling gas to the bearing. The openings may bepositioned on a circle with a radius greater than the radius of theouter circle. The openings may be positioned in the gaps betweenadjacent foils.

The peaks of the ridges in each of the bump foil tabs may be generallyaligned and collinear. Alternatively, the peaks of the ridges in one tabmay be aligned with the centerlines of the flats in adjacent tabs, or,equivalently, the centerlines of the flats in one tab may be alignedwith the peaks of the ridges in adjacent tabs. The peaks of all of theplurality of ridges of the bump foil tabs may be of the same height ormay be of differing heights.

The top foils may be coated with a hard lubricious layer to minimizewear during startup and shut down of a machine when the bearing runnerwill contact and rub against the top foil.

A second embodiment, may incorporate all of the bump foils and all ofthe top foils in individual planar sheets, a bump foil sheet and a topfoil sheet.

The bump foil sheet may be stamped and/or pierced, to create a number ofspaced-apart circumferentially-arranged bump foils, each with four edgesand each of the bump foils being unsecured on three edges and continuouswith the sheet on its fourth edge. The bump foils, as in the firstembodiment, may have the general configuration of annular arcs with thefoils disposed about a bump foil center.

The top foil sheet may be stamped and/or pierced, to create a number ofspaced-apart circumferentially-arranged top foils, coextensive with thebump foils, each with four edges and each of the top foils beingunsecured on three edges and continuous with the sheet on its fourthedge. The top foils, as in the first embodiment, may have the generalconfiguration of annular arcs with the foils are disposed about a topfoil center.

The foil thrust bearing may then be assembled by assembling the bumpfoil sheet to the thrust plate and overlying the bump foil sheet withthe top foil sheet, ensuring that the centers of the thrust plate, bumpfoil sheet and top foil sheet coincide and the that the top foilsoverlie the bum foils. The top foil and bump foil sheets may be attachedto the thrust plate in any convenient fashion but welding is preferred.

The details of the top foils and bump foils of the second embodiment mayparallel those of the first embodiment. Also, a plurality of openingsfor ingress of cooling gas, similarly positioned as in those of thefirst embodiment, may be formed in at least the thrust plate and, ifrequired, in the top foil and bump foil sheets.

The thrust bearings of both the first and second embodiments may besplit along a line passing through the center of the thrust plate andpassing through the gaps between the foils for bearing disassembly andreassembly. Guidance features may be incorporated into the thrust platefor ease of alignment during reassembly.

Other illustrative embodiments of the invention will become apparentform the detailed description provided hereinafter. It should beunderstood that the detailed description and specific examples, whiledisclosing embodiments of the invention are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in quarter cut-away perspective view a first embodiment ofa micro-machine capable of high rotational speeds comprising a singlepiece hollow rotor with integral turbine wheel suitable for impartingrotation.

FIG. 2 shows the embodiment of FIG. 1 in cross-section to betterillustrate the placement of the thrust and journal bearings and themanner in which air is bled from the turbine exhaust for bearingcooling.

FIG. 3 shows in partial cut-away perspective view the embodiment of FIG.1 to better show some additional features, particularly flow paths forcooling air.

FIG. 4 shows a foil journal bearing adapted for receiving anddistributing cooling air flow.

FIG. 5 shows further detail of attachment means for the bump foil andFIG. 6 shows further detail of attachment means for the top foil in thebearing of FIG. 4.

FIG. 7 shows the complementary top foil bearing shell attachment featureof the bearing of FIG. 4.

FIG. 8 shows, in partial cut-way, a plan view of foil thrust bearingwith a slotted bump foil for improved bearing compliancy and cooling.

FIG. 9 shows a cross-sectional view of a portion of the foil thrustbearing of FIG. 8.

FIG. 10 shows a fragmentary detail of flange 15 of FIG. 1 to betterillustrate features for capture and retention of machining debris.

FIG. 11 shows, in quarter cutaway perspective, a second embodiment ofthe invention.

FIG. 12 shows a second embodiment of the invention in cross-section.

FIG. 13 shows a fragmentary view of the abutting faces of two elementsof an assembled rotor suitable for application in the second embodimentof the invention.

FIG. 14 shows a means by which the magnets may be assembled to andretained in a one-piece rotor to enable its use in a second embodimentof the invention.

FIG. 15 shows a first embodiment of a foil journal bearing adapted togenerate increased hydrodynamic pressure.

FIG. 16 shows the top foil configuration and the generated hydrodynamicpressure profile of the foil journal bearing of FIG. 11 duringoperation, and compares its top foil configuration and hydrodynamicpressure profile during operation with that of a more conventional foiljournal bearing.

FIG. 17 shows a second embodiment of a foil journal bearing adapted togenerate increased hydrodynamic pressure.

FIG. 18 shows a third embodiment of a foil journal bearing adapted togenerate increased hydrodynamic pressure and a second embodiment of acooperative foil bearing shell retainer groove geometry.

FIG. 19 shows a fourth embodiment of a foil journal bearing adapted togenerate increased hydrodynamic pressure and a second embodiment of acooperative foil bearing shell retainer groove geometry.

FIG. 20 shows a fragmentary view of an alternate embodiment of a bumpfoil.

FIGS. 21, 22 and 23 show exemplary composite bump foils fabricated fromtwo individual bump foils.

FIG. 24 shows, in exploded perspective view, an embodiment of a foilthrust bearing.

FIG. 25 shows in fragmentary perspective view an embodiment of a bumppad similar to that shown in FIG. 24.

FIG. 26 shows a first preferred embodiment of a bump foil pad.

FIG. 27 shows the bump foil profile of taken along line QQ shown in FIG.26.

FIG. 28 shows the streamlines of fluid flow over an operating foilbearing pad.

FIG. 29 shows the streamlines of fluid flow over an operating foilbearing pad when radially-inflowing pressurized air is introduced at theouter diameter of the bearing pad.

FIG. 30 shows a second embodiment of a foil thrust bearing.

FIG. 31 shows an embodiment of a bump foil pad adapted for mechanicalattachment to a thrust plate.

FIG. 32 shows an embodiment of a bump foil pad for promoting developmentof a turbulent boundary layer on the top foil during operation.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in quarter cut-away perspective view, a first embodimentof a micro-machine 10, capable of high rotational speed and comprising asingle piece hollow rotor with integral turbine wheel suitable forimparting rotation. In one embodiment the components of themicro-machine may be constructed and arranged so that themicro-machining center is capable of high operating speeds of greaterthan 700,000 rpm, or greater than 1,000,000 rpm or up to 1,500,000 rpm.

The micro-machine employs both journal foil bearings 28, 30 and thrustfoil bearings 60, 62 (best seen at FIG. 2). Foil bearings support theshaft on a self-generated film of air, so that, at operating speeds, ineither a journal or thrust bearing, there is no contact between theshaft and the bearing. Also because of the low viscosity of theoperating fluid (air or any operating process gas or liquid), frictionallosses are lowered and temperature rise, though not insignificant, isinherently lower than most liquid lubricated bearings. Thus issues ofthermal expansion and its influence on shaft-bearing tolerances arereduced.

Foil bearings, for example foil journal bearing 100 shown in FIG. 4,typically include three major elements: a smooth, thin top foil 108which provides a smooth bearing surface; a corrugated compliant supportfoil or bump foil 106 which underlies the top foil and providesresilient support to the top foil; and a supporting shell 102 whichpositions and secures the foils.

As described in greater detail later, interaction between the top foil108 and the rotating shaft 112 generates the air film which supports theshaft, while the corrugated compliant bump foil 106 contributes bothstiffness and damping to the bearing. The pressure supporting the shaftload is conveyed by the air film to the smooth top foil 108 whichdeflects and elastically deforms the corrugations of the bump foil 106,and thereby imparts stiffness to the bearing. Also, the peaks 106′ ofthe ridges of corrugated bump foil 106 are in contact with the undersideof the top foil 108 while the valleys 106″ of the bump foil aresupported by the inner surface of bearing shell 102. The geometry of thecorrugations assures that as the corrugations are displaced verticallythey will simultaneously spread laterally. Hence the peaks of thecorrugations will rub against the underside of the top foil, and valleysof the corrugations will rub against the inner surface of the supportingshell. The friction associated with the rubbing of the foil willdissipate energy and impart damping to the bearing.

Bearing stiffness and damping is important in micro-machine applicationsbecause rotating machine tools such as mills, burrs or drills generatecomplex, time-varying, three-dimensional loads even under invariant orsteady-state cutting conditions. Cutting loads may also change abruptly,for example when the tool enters or exits a cut. Thus micro-machinebearings must be selected to provide sufficient stiffness and damping toaccommodate both steady state and transient loads without generating aninstability or excessive deflection.

Even with air as an operating fluid, operating speeds of up to 1,500,000rpm may result in some increase in bearing temperature. Since foilbearings employ compliant elements they may be made more tolerant ofthermal expansion or of shaft-bearing misalignment than rolling contactbearings, but only at the expense of reduced bearing stiffness anddamping. It may therefore be preferred to locate and position bearingsto minimize bearing misalignment and to apply enhanced temperaturemanagement strategies to the bearings to minimize thermal effects.

The supporting air film is self-generated, resulting from the relativemotion of the shaft and the bearing. The ability of the bearing tosupport the load imparted by the shaft depends on the relative motion ofshaft and bearing and only after the shaft is rotating rapidly is theair film capable of fully supporting it. Hence during periods of lowshaft rotational speeds, for example during start-up and shut-down, theshaft may contact, rub on, and wear the bearing surfaces, potentiallylimiting useful bearing life. The bearing surfaces, particularly the topfoil surfaces which may contact the moving shaft, may therefore becoated with a wear-reducing surface coating. The coating may be bothhard and lubricious. A suitable coating may be Korolon™ 1350, aproprietary, spray-gun-applied nickel-chrome coating with solidlubricants developed by MiTi, Albany, N.Y.

Referring to FIGS. 1-2, the micro-machine 10 may include a hollow rotor12 connected to an extended overhung shaft 14 by means of flange 15which has an outer surface 16 and an inner surface 16′. Flange 15 may bean integral part of rotor 12 with each machined from a common stock ormay be attached to the front end of rotor 12 by suitable means includingwelding, brazing and mechanical fasteners such as screws 18 (FIG. 2)which pass through holes 17 to engage threaded holes on rotor 12 (notshown). Shaft 14 is adapted to incorporate tool holder mechanism 20 forsupport and releasable retention of tool 22. Tool holder mechanism 20may be a cylindrical cavity of precise dimension intended for shrink-fitretention of shank 23 of cutting tool 22.

The rotor 12 may include integral turbine wheel and thrust disc 42 (bestseen at FIGS. 2 and 3) driven by a pressurized gas or gas mixture,including air, as a source of power. The pressurized gas may beintroduced at nozzle ring 24, where by means of guide-vanes 82(indicated in FIG. 2) the flow is converted to supersonic jet-streams,and then directed at a series of reaction turbine blades 40 (FIG. 3)mounted on turbine wheel/thrust disc 42 (FIG. 2).

Turbine wheel/thrust disc 42 has an axis of rotation 36 which isgenerally coaxial with the centerline of rotor 12. The thrust discportion of turbine wheel/thrust disc 42 may preferably be coated with awear resistant coating including, for example, thin, dense triobologicalchromium alloys, titanium nitride and others. The coating may be bothhard and lubricious. As with the journal bearings the proprietarynickel-chrome coating with solid lubricants, Korolon™ 1350 developed byMiTi, Albany, N.Y. may be suitable. The direction of rotation for theturbine blade configuration shown is indicated by arrow 37.

Rotor 12 may be supported by two foil journal bearings 28, 30, mountedinside machine 10 split housing 50, comprising first portion 52 andsecond portion 54 as shown best at FIG. 2. The foil journal bearings,which incorporate several novel features and will be described morefully later, may have very thin foil retaining shells (102, FIG. 4)comparable to the thickness of the smooth top foil (108, FIG. 4). Thinbearing shell 102 need not be a cylinder as shown in FIG. 4, but mayinstead be a wrapped foil in a quasi-cylindrical shape. After insertionof compliant elements, bump foil 106 and top foil 108, the completejournal bearing may be positioned, as bearings 28, 30 in split housing50. An analogous construction, described in detail later, may beemployed for the thrust bearings 60 and 62, arranged in opposition toaccommodate axial loads applied to spindle 12.

The relative thickness and stack-up heights of the foil bearingcompliant elements are extremely small relative to the thickness ofsplit housing 50. The stack-up height may range from a few micrometersup to 1.25 mm, while the wall thickness of the machines' bearing housingmay range from a few mm up to 10 cm or more, depending on the size ofthe machine. The bearings shown have a thin (bearing) shell relative tothe wall thickness of split shell 50. But it will be appreciated thatwith suitable adjustment to the geometry of split shell 50, in ways wellknow to those skilled in the art, thick shell bearings like those shownin FIG. 4 may also be employed.

The journal bearings may be positioned on either side of turbinewheel/thrust disc 42, that is one bearing is positioned within theinterior diameter of each of first 52 and second portions 54 splithousing 50. The bearings are suitably dimensioned to accommodate theouter diameter of hollow rotor 12.

Rotor 12 may be supported by a split housing 50 comprising a firstportion 52 and a second portion 54. Portions 52 and 54 may be releasablyattached, for example with mechanical fasteners, along a commonattachment plane generally positioned on the mid-plane of the turbinewheel/thrust disc 42.

Opposed thrust bearings 60 and 62 (for clarity shown only in FIG. 2) maybe mounted in recessed openings 61 and 63 (FIG. 2) of housings 52 and54, and, more particularly within rings 24 and 32, respectively. Opposedthrust bearings 60 and 62 will accommodate axial loads, that is loadsapplied along the rotation axis 36 and directed either toward or awayfrom tool 22. With this configuration the micro-machine may be capableof operation in all orientations and attitudes.

The micro-machine may be assembled with the following procedure. Rotor12 (after assembly to flange 16 and overhang 14, if required) may bebalanced to 0.1 microgram-meter (μg-m) for a 6 to 8 mm shaft, 0.3 to 0.6(μg-m) for a shaft with a 4 mm diameter with a micro-balancing machine.Thrust bearing 60 may be positioned on ring 32, possibly in a mountingrecess, not shown, of housing 52. The rotor may then be advanced intohousing 52 containing foil journal bearing 30 in a directioncorresponding to ‘A’ shown on FIG. 2 until turbine wheel/thrust disc 42just contacts thrust bearing 60. Thrust bearing 62 may be inserted inrecess 63 of ring 24 of housing 54. Housing 54 containing foil journalbearing 28 may then be inserted over rotor 12 in a direction shown as‘A’ in FIG. 2 until turbine wheel/thrust disc 42 just contacts thrustbearing 62.

Housings 54 and 52 may then be releasably attached, for example throughthe use of bolts (not shown) inserted into hole 70 which engage thethread in aligned threaded hole 72. A V-clamp, sized to engage thecylindrical and end surfaces of flanges 24 and 32, may also be used.Generally, separate and independent alignment and attachment featuresmay be employed for housings 52 and 54. Suitable alignment features mayinclude mating features such as dowel pins on one housing engagingmating holes on the second housing (not shown).

Housing surfaces 28′ and 30′ support journal foil bearings 28 and 30respectively. The relative alignment of all the bearings, butparticularly of the journal bearings 28 and 30 will depend on thealignment, both angular and positional, achieved between housings 52 and54. Because of the inherent compliance afforded by the foil bearings,both thrust and journal, some misalignment of the housings may betolerated. However it will be appreciated that compliant element foilbearings' internal components may be extremely thin with a total stackheight of only 0.01 to 0.02 inches so that any misalignment of housings52 and 54 is likely to be minor.

More generally, it will be appreciated that the housings must beassembled and arranged to at least not exceed the maximum allowablebearing tolerance. Inasmuch as some of the maximum allowable bearingtolerance will be required to accommodate the dimensional changesundergone by the bearing as it expands due to temperature rise in use,in one embodiment the housing misalignment should preferably bemaintained at no more than half of the total allowable tolerance.

The temperature rise of the bearing and thus the dimensional changesundergone by the bearing in use may be minimized by provision offeatures to promote enhanced cooling. A representative foil journalbearing adapted for such enhanced cooling is shown in FIGS. 4, 5, 6 and7. FIG. 4 shows a foil journal bearing 100 with bearing center 110 incross-section. The bearing may include a bearing shell 102 with features104 such as one or more notches or grooves formed in an inner surfaceand adapted for retention of the bump foil 106 and top foil 108. Topfoil 108 may incorporate features 105, for example a tongue constructedand arranged to be generally complementary to shell feature 104,intended to compliantly engage shell features 104 and thereby retain thetop foil under rotation of shaft 112 about center 114 in the directionindicated by arrow 111.

Bump foil 106 may have a similar retaining feature 105′ to feature 105of top foil 108, as shown in fragmentary view FIG. 5 compliantlypositioned and restrained between features 105 and 104. Alternatively,bump foil 106 may be secured by spot welding, as at 109 to shell 202(FIG. 11). It will be appreciated that these attachment methods arealternate methods and are not be employed in conjunction. As shown inFIG. 6 shell retaining feature 105 may incorporate slots 107 or similarfeatures. Such features may be incorporated to maintain a more constantcompliance of the top foil by at least partially offsetting thegeometric stiffening resulting from introducing shaped retaining feature105 and impart axial compliance to the bearing. These features areeffective in imparting a self-alignment capability to the bearing toaccommodate minor axial shaft misalignment.

The inclusion of grooved features 104 in bearing shell 102 may also beeffective in promoting improved ingress of cooling air, and facilitatesits distribution within the foil bearing to promote improved cooling.The process may be made even more effective by ‘damming’ or obstructingone end of groove 104 to induce circumferential airflow as illustratedin FIGS. 6 and 7 which illustrates airflow in the direction of arrow 118entering groove 104 at a first end 115 and exiting at second end 116. Byblocking end 116, by a bent-up feature on top foil 108, or by forminggroove 104 only partway into the bearing shell to create, by means ofthe remaining shell portion, an endwall at 116 or by any other meansknown to those skilled in the art, incoming airflow 118 may beredirected circumferentially by means of slots 107 (FIG. 6) indirections shown by arrows 119 and 119′ to more fully participate incooling the journal bearing.

The benefits of such cooling may be appreciated by consideration of therelative locations of bearing center 110 and shaft 112 center 114.Because the bump foil is compliant it may flex and displace when loadedby the film of air on which the shaft is supported. In operation,bearing heating may result in an increase in the shaft or journaltemperature causing it to expand and more closely approximate thediameter of the bearing shell. Thus the greater the temperature rise ofthe journal the greater the initial clearance and the greater theinitial compliance which must be designed into the bearing, therebycompromising bearing stiffness and degrading the micro-machine accuracy.Hence, as will be discussed in greater detail in subsequent sections itmay be preferred that all bearings be gas-cooled and that the bearings,as indicated in the exemplary design of FIG. 4, be adapted to moreefficiently receive and distribute the cooling gas.

Similar considerations apply to foil thrust bearings, a representativeexample of which 120 is shown in partial cut-away plan view in FIG. 8and in selected cross-section 9-9 in FIG. 9. Here, foil thrust bearing120 may include bearing plate 122, suitably positioned with respect toshaft 130 rotating about its center 131 in a direction indicated byarrow 137. Bearing plate 122 may be overlaid by a plurality of bumpfoils, an example of which is shown as 126 and by a foil sheet 138 whichhas been slit and formed to develop a plurality of top foil segments 128separated from foil disc 138 along a three-sided path shown as 136 anddeformed to create an elevated lip 125 along its remaining line ofattachment to foil disc 138. Bump foil 126, as best shown in FIG. 9,comprises a series of ridges 129 of varying heights separated by flats127. The designators ‘a-g’ associated with the ridges of cross-section 9correspond to the ‘a-g’ designators associated with bump foil 126 inFIG. 8. Foil 126 may include a plurality of circumferentially-orientedslots 135 to promote improved gas flow within the bearing and improvedbearing cooling. Other features of foil thrust bearings adapted for usein micromachining centers will be addressed in a subsequent section.

Returning to FIG. 2, ring 24 may receive pressurized air flow (or otherprocess gas) through inlet port 80 which discharges at a higher, evensupersonic speed after passing through a series of guide-vanes 82. Thedeveloped jet flow may be directed toward reaction turbine blades 40where it may transfer its kinetic energy to the blades to inducerotation of the rotor 12. After interacting with the turbine blades theprocess gas may expand and cool and primarily discharges into annularexit port 84.

As will be appreciated from the prior discussion of the characteristicsof foil thrust bearings, clearance exists between the backing plate andbump foil/top foil combination of a thrust bearing. Thus the pressurizedgas of annular exit port 84 can bleed from the outer radius of thrustbearing 60 to its inner radius where it may then be constrained to flowthrough foil journal bearing 30 and between rotor 12 and housing 52. Theoverall flow path is indicated at 90 (FIG. 3). After traversing thelength of housing 52 the gas flow may exit at end 53 of housing 52. Aslinger washer (not shown) may be located at the point of exit toredirect the gas flow away from the cutting tool to minimize airbornedebris. By this scheme of flow passages the bearings may be both cooledand kept free of external contamination, such as the debris generatedduring cutting.

A parallel flow scheme, providing equivalent benefits, and shown as 92in FIG. 3, may be followed for foil thrust bearing 62 and foil journalbearing 28. Here however there is no corresponding plenum to 84 and flowthrough the bearings simply results from the small but necessaryclearances between the rotating turbine wheel/thrust disc 42 and thestationary flow shaping features of ring 24. Provision may also be madefor bleed-off of excess gas if flow paths 90 or 92 become obstructed andcreate excessive back pressure. Openings 74 and 74′, located on rings 32and 24 respectively, may create pathways for release of vent flows 76(FIG. 3) and 76′ (FIG. 2).

Flow passages 90 and 92 assure that the thrust bearings 60 and 62 willnot be deprived of cooling air as long as the rotor is rotating. Thusthere will always be cooling gas, which may also serve as a lubricant,available for the bearings. An additional benefit is that the gas flowmay serve to exclude debris from the bearings.

Another means of managing machining debris is indicated in FIG. 1 whichshows a series of channels 21 extending through flange 16 into theinterior of rotor 12. These channels may extend radially outward fromsurface 16 to surface 16′ of flange 15 and are inclined to rotation axis36 as may best be seen in FIG. 10. Thus the openings 21′ of channels 21on surface 16 may be located on the circumference of a first circle 37on face 16 and the channels 21 may extend radially outward and at someinclination to rotation axis 36 to form openings 21″ on surface 16′,where openings 21″ may be located on the circumference of circle whosediameter is greater than that of circle 37.

Under rotation, this channel 21 configuration may act as a scoop andensures that any debris entering openings 21′ may be transported throughchannels 21 and deposited within hollow rotor 12, thereby minimizingairborne debris.

A second embodiment of the invention is shown in partial cutawayperspective in FIG. 11 and in section in FIG. 12. In this embodiment thehousing may be split longitudinally, that is along the axis of rotationof the rotor.

Micro-machine 140 may include a rotor assembly 142 adapted toaccommodate a cutting tool (not shown) in toolholder portion 151,comprising toolholder cavity 150. Rotor assembly 142 may be an assembledmultipiece rotor comprising permanent magnet motor rotor 144, toolholderportion 150 and impeller attachment portion 153 with all three piecessecured and attached to one another through shrink-fitted sleeve 158. Asshown, radial flow compressor impeller 146 may be a separate elementattached to rotor assembly 142, specifically to impeller attachmentportion 153 for example by mechanical fastener 148. However radial flowcompressor impeller 146 and impeller attachment portion 153 may also befabricated as a single piece. Stator 160 may be incorporated in splitmachine housing 165 and positioned to cooperatively interact withpermanent magnet motor rotor 144 to induce rotation of rotor assembly142.

Rotor assembly 142 may be supported on split journal foil air bearings154 and restrained from motion along the direction of rotation axis 152by housing-mounted, opposed thrust bearings 156, 156′ acting againstrotor disc 157. Cooling gas inlets 164, 166 may be provided to directpressurized cooling gas to journal bearings 154 (inlet 164) and tothrust bearings 156 (inlet 166). After passing over the bearings thecooling gas may be discharged at outlet 162.

Pressurized cooling gas may be derived from any convenient source. Themicro-machine shown may be capable of providing pressurized air withoutrecourse to an external source. Here, incoming air flow 170 induced byrotation of impeller 146, passes through air passage 171, may becompressed by cooperative interaction of impeller 146 and the shapedinner surfaces of air passage 171 and discharged through ducts 172 intostorage tank 174 where it may be accessed at outlet 176 and fed throughcooling ports 164 and 166 in controlled fashion.

Rotor assembly 142 has been described as a multipiece rotor comprisingtoolholder portion 149, impeller attachment portion 153 permanent magnetrotor 144 which may be permanently attached using a shrinkfitted sleeve.As depicted the various elements are shown in butt-joint configurationso that only the frictional interaction between the sleeve and theindividual elements enables torque transmission from one element toanother. Another approach may be to incorporate complementary featureson the abutting members to improve the mechanical interlock. An exampleis shown in FIG. 13 which, without limitation, shows permanent magnetrotor 144 with slot 180 and toolholder portion 151 with complementarykey 181. When properly aligned, key 181 will tightly engage slot 180when face 182 abuts face 183. Thus the mechanical engagement of key 181and slot 180 will be effective in transmitting torque while ashrinkfitted sleeve may overlie shaft surfaces 184 and 185 and hold themin longitudinal alignment. Of course, more complex mechanically-engagingfeatures than the slot and key configuration shown may also be adopted.These may include configurations which also tend to axially align theshaft surfaces 183, 184.

It may also be possible to fabricate rotor assembly 142 as a one piecenon-magnetic shaft, not incorporating compressor impeller 146,comprising slots or pockets for incorporation of magnets for the rotorand a short shrink-fitted sleeve to aid in magnet retention underrotation. Such a configuration is shown in FIG. 14 which may show asegment of rotor assembly 142 which may have been suitably pocketed orslit at locations 190 to accept appropriately oriented magnets which maybe retained at least by shrink-fitted sleeve 158, and may besupplemented by adhesive or other retention means.

Fabrication of the micro-machine may include; finishing the assembly ofthe rotating group first; conducting final machining/polishing andbalancing to achieve acceptable rotor dynamic behavior; positioning therotor in bearings; positioning the rotor and bearings in one of theparts of the split housing; and finalizing assembly by positioning andreleasably attaching the remaining parts of the split housing. Theassembly may be performed in this sequence to ensure acceptable rotordynamic behavior which is not achievable if the rotor is not balanced asa complete assembly. It will be appreciated by reference to FIGS. 11 and12 that this assembly sequence may be facilitated by the journal andthrust bearings being split bearings. Details of the split bearingdesigns which may be employed will be discussed in a later section.

The assembled, complete rotor, as shown in FIGS. 11 and 12, may bedriven by the electric motor comprised of permanent magnet rotor 144 andstator 160, which together form a brushless DC electric motor. As iswell known to those skilled in the art, the stator of such a motorcomprises electrically conducting coils energized by an electroniccommutation controller system (not shown). Stator 160, like thebearings, may be split along the split axis or axes of the housing,requiring that electrical connection (not shown) be made between thewindings associated with the split portions of the stator wiring formotor operation. Upon reaching its maximum operating speed compressorimpeller 153 in cooperation with the shaped interior surfaces of airpassage 171 produces pressurized air to greater than ambient pressurewhich may be temporarily stored in tank 174. When required thepressurized may be directed through exit port 176 to the bearings andthe motor rotor and stator in order to minimize any temperature rise.After cooling the machine elements the air may be vented at port 162,though it may be preferred that some air bleed past the clearancebetween tool holder portion 149 and housing 165, and thereby be directedtoward the work piece, in order to remove the debris and cool thecutting edges of tool 151.

Thrust bearings 156 and 156′ may be cooled from their outer diameterstoward their inner diameters by directing cooling flow from port 166.This is effective in enhancing the cooling because it is in oppositionto the flow of frictionally-heated air impelled by centrifugal forceimparted by rotor disk 157 toward the outer diameter. By directingcooling air flow, in the radially inward direction, the two opposingfluid flows are in “counter-flow” configuration which maximizes heatexchange and more effectively cools the thrust bearings.

The micro-machine design shown in FIGS. 11 and 12 may, with minormodification be adapted to operate on pressurized gas and therebyeliminate the electric motor. In a first design variant the motor may beoperated on pressurized gas introduced at air passage 171. Bymodification of compressor impeller 146 it may be adapted to function asa radial turbine inducing rotation of rotor assembly 142 and dischargingair, of still at greater than atmospheric pressure, into tank 174.Alternatively the machine may be constructed with the drive systems,electric and gas shown and used either in either mode.

In a second design variant an independent turbine may include animpeller suitably surrounded by gas flow shaping surfaces formed inhousing 165 may be located on and coupled to rotor assembly 142 andoperated by pressurized gas. No other modifications need be made to themicro-machine. In this design variant the gas discharged from theindependent turbine may simply be vented and the cooling air stored anddischarged from tank 174 may be generated by impeller 146 as previouslydescribed.

Foil journal and thrust bearings adapted for improved cooling andthereby suited for use in micromachining centers have been previouslydescribed. These bearings may incorporate other novel features asdescribed below.

Another embodiment of a foil journal bearing 200 may be shown in FIG.15. In this design, each of a plurality of top foils 208 may be securedto a split housing comprising (housing) shell elements 202 and 203 byspot welds 109. In common with the bearing 100 of FIG. 4 the top foilsof bearing 200 may overlie a number of suitably-secured corrugated bumpfoils 206. Bump foils 206 may be secured to the housing 202 by spotwelds 109, and compliantly distance top foils 208 from the innerdiameter of housing 202. It should be noted that bearing housing 202 maybe split along line x-x and readily and consistently re-assembled. Shellelements 202, 203 may include complementary alignment or engagementfeatures such as guide pins 220 fitting into reamed holes 221, orshoulder screws engaging complementary partially-threaded holes (notshown) to ensure alignment. Alternatively alignment may be achievedthrough dimensional control and alignment of the bearing supportfeatures in a split housing.

Top foils 208 may circumferentially extend on either side of theirmounting locations and have both a trailing edge segment 208′ and aleading edge segment 208″ relative to their attachment location. Asshown, each top foil 208 necessarily comprises a leading edge and atrailing edge. With appropriate modification to the top foil 208mounting and retention procedure, top foil lengths corresponding to theleading edge length and to the trailing edge length may be independentlymounted adjacent to one another without prejudice to their performance.However, there may be a definite relationship between the bump foilstrips and the top foil and in the way they are anchored to the bearingshell. As shown in FIG. 15, attachment points 109 may always be locatedin the vicinity of the trailing end of the uppermost of the overlyingtop foil segments. This may be done to systematically vary thestructural stiffness of each foil segment from leading end to trailingend of top foil 108.

As shown in FIG. 15, the lengths of trailing 208′and leading 208″ edgesegments may be unequal but, in a design with N foils their lengths areso chosen as to enable the trailing edge of a first foil to overlie theleading edge of a second foil and the trailing edge of a third foil tooverlie the leading edge of a third foil. This relative foil placementcontinues around the inner circumference of the bearing housing untilthe trailing edge of the Nth foil overlies the leading edge of the firstfoil, thereby extending the composite top foil surface around the entireinner circumference of the housing as shown by the four-top-foilconfiguration shown in FIG. 15.

In operation, the shaft's surface is initially in rubbing contact withthe top foil surfaces until, with increasing shaft rotation speed a thinhydrodynamic film develops between the shaft and top foil surface. Theshaft is then levitated from the bearing's surface and separated from itby an air film.

The top foil 208 may be free, subject to any restoring forces exerted bycompliant bump foil 206, to bend and pivot about the center of thebearing mounting groove 204, responsive to the influence of the dynamic,static or thermal movements of the shaft with respect to the bearing.Top foil 208 may also deform elastically. And such elastic deformationmay be local. For example the trailing edge 208″ of a first top foil,partially supported by the leading edge 208′ of a second top foil asshown in their unloaded configuration in FIG. 15 may locally deform asshown in the fragmentary view of bearing 200 shown in FIG. 16 to createa stepped bearing surface at 209. This stepped bearing surface 209creates an “elasto-pressure dam” which is effective in enhancing thehydrodynamic pressure profile resulting from the shaft-bearinginteraction enabling higher bearing loads and more rapid development ofthe hydrodynamic film.

In one embodiment, curve 240 (FIG. 16) shows the pressure versusdistance, measured along the bearing, developed by shaft 212 rotating ina direction indicated by arrow 224 for a top foil deformed to developfeature 209. The top foil configuration shown may be typical, forexample, of the configuration which would be adopted by the bearing ofFIG. 11 under the same conditions. FIG. 16 also shows, as curve 230 thepressure versus distance profile which may be developed by a moreconventional top foil configuration 219, shown in ghost. The increase inhydrodynamic pressure generated by the elasto-pressure dam according toone embodiment may be identified by the cross-hatched area 235representing the difference between the hydrodynamic pressure profiles.The elasto-pressure dam may also increase damping and improve bearingstability and provide pressure variation that is effective in pumpingadditional air through the bearing for more efficient bearing cooling.

The novel stepped bearing surface may also accommodate two-phase(gas-liquid) flow, for example compressed air with entrained waterdroplets. In conventional foil journal bearings, any liquid mixed withthe gas vaporizes. Because the mixture is heated by passage through thebearing clearance it results in a significant volume expansion andprecipitates a rapid pressure rise, which if severe may interfere withproper bearing operation. But a compliant elasto-pressure dam mayrespond to the localized rise in pressure by deforming further, therebyrelieving the pressure increase and promoting stable bearing operation.

Other embodiments are also within the scope of the invention. Thestructural compliance of foil bearings, as described here, may beestablished through the interaction of the top foil with the underlyingbump foil, which is deformed or corrugated to a form comprising analternating series of ridges and flats. These bump foils 106, 206, asshown in FIGS. 4 and 15 are anchored at one end and free at the otherend and thus may slide circumferentially, and provide damping, whenpressure is applied radially, for example, if the shaft is displacedfrom its original central position toward the bearing housing wall.

Generally the bump foil 206 height or the change in height betweenadjacent ridge tops and flat bottoms in the bump foil is constant andindependent of position in the foil, leading to a uniform elasticresponse at any location along their length. However, as illustrated inFIG. 16 the elasto-pressure dam may arise in response to a localgeometric irregularity 209. In the case shown in FIG. 16 geometricirregularity 209 is attendant on the geometric discontinuity resultingfrom the overlapping top foil geometry, where portion 208″ overliesportion 208′ of the adjacent top foil. But similar results may beobtained through the use of a bump foil whose compliance varies withposition in a predictable manner.

A suitable configuration is shown in FIG. 17 which shows a foil journalbearing 250 with a single foil 260 with a retaining feature 255 engagingcomplementary groove 254 in housing 252. Foil 260 may comprise a first,partially corrugated portion of foil 261 in contact with the innerdiameter of shell 252 and approximately equal in length to thecircumference of the inner shell diameter; and a second, planar portion262 again of length approximately equal to the circumference of theinner shell diameter, overlying portion 261. Foil 260 is shown in itsoperating configuration.

At locations 264, topfoil 262 is unsupported by any corrugations likethose at locations 266. At locations 264 therefore topfoil 262 mayadopt, as shown, a configuration similar to that shown at 209 in FIG.16, that is, it may exhibit a configuration like that of theelasto-pressure dam described previously (209 at FIG. 16).

Similar performance may be obtained with other top foil and bump foilconfigurations. For example in FIG. 18, bearing 270 may comprise aseparate bump foil 276 secured at retainer groove 274′ in housing 272,again exhibiting corrugated and non-corrugated portions and a separatetop foil 278 secured at retainer groove 274 in housing 272. In anotherexample foil journal bearing 280, shown in FIG. 19, a single foil 281may comprise a bump foil 286 exhibiting corrugated and non-corrugatedsections and a smooth top foil 288 which overlies it. The individualsections are disposed on either side of retainer groove 284 located inhousing 282 and complementary foil retaining feature 285. Bump foilsegment 286 extends in counterclockwise orientation and underlies topfoil section 288 which extends clockwise. It will be appreciated thathousings 252, 272 and 282 for the journal bearings shown in FIGS. 17, 18and 19 may also be split for ease of assembly and disassembly. In thiscase, the bump foils and top foils may be unwrapped from the shaft forbearing disassembly or wrapped around the shaft for assembly.

In FIG. 19, the retaining groove 282 and complementary retaining feature285 are not trapezoidal as previously shown but generally resemble theupper case Greek letter omega (Ω). It will be appreciated that thespecific design approaches described and depicted are illustrative andnot limiting. A variety of tongue and groove or mating or nestedretaining features are within the scope of the invention.

In other select embodiments the longitudinally-varying bump foilgeometry of FIGS. 17-19 need not symmetrically increase and decrease butmay slowly ramp up and rapidly decrease as shown in the partial bumpfoil segment 296 shown in FIG. 20. The ability to systematically varybump foil stiffness is not limited to only the designs shown but may beemployed in any foil journal bearing including the designs of FIGS. 4and 15. Finally, the bearing may employ more than a single bump foil sothat the bump foils will act cooperatively. Exemplary configurations areshown in FIGS. 21, 22 and 23.

In FIG. 21, portions of two identical bump foil segments 306 and 306′,are shown arranged in opposition to cooperatively deform under allloadings and, in combination form composite bump foil 307.

In FIG. 22 two bump foils 316 and 316′ of varying height are nested toform composite bump foil 317. Bump foil 317 enables abruptly varyingbump foil stiffness with displacement since bump foil 316′ willcontribute to the over bump foil stiffness only after bump foil 316 hasbeen deflected by an amount δ as shown in FIG. 22. It will beappreciated that the spacing between individual ridges, shown asconstant in FIG. 22 may be varied as in FIG. 23 to enable moreprogressive stiffness variation with displacement. In fragmentary viewin FIG. 23, nested individual bump foils 326 and 326′ comprise compositebump foil 327. In this configuration however the separation between thebump foils may vary. Thus at location 324 the maximum separation is δ₁,at location 323 the maximum separation is δ₂ and at location 322 themaximum separation is δ₃. Because δ₁<δ₂<δ₃, the stiffness of compositebump foil 327 will progressively increase with displacement.

All the composite bump foil configurations shown were fabricated fromtwo individual bump foils. This is not intended as a limitation and itis recognized that the concepts may be readily extended to comprehendmore than two individual foils.

One embodiment of a foil thrust bearing design 400 is shown in explodedperspective view in FIG. 24. Thrust foil bearing 400 may include anunderlying thrust plate 402, bump foil sheet 404 and an overlying topfoil sheet 406 positioned and located axially about centerline 408,which is the axis of rotation of runner 410 (shown in ghost) rotating ina direction indicated by arrow 412. Bump foil sheet 404 may include aplurality of equally-spaced corrugated annular segments 414 each ofwhich may be overlain by a top foil pad 416 bounded by three slits 417,418 and 419 and may remain secured to top foil sheet 406 only along line420. Pad 416 may therefore be free to flex and pivot about line 420 inresponse to any applied load with a component directed along centerline408. In similar manner to the foil journal bearing, because pad 416 mayelastically deform under load, the shape which it adopts will bemoderated by the corrugated foil segment 414 which underlies it. Againcorrugated bump foil pad 414 should be suitably profiled to induce inthe top foil a preferred configuration when under load.

The corrugated bump foil pads 414 may be fabricated as individualcorrugated segments and attached directly to thrust plate 402—examplesof this configuration will be shown later. The configuration shown inFIG. 18 may however be formed in similar fashion to that employed toform the top foil pads 416. Thus, as shown in FIG. 25, bump foil sheet404′ may be shaped and formed by stamping then slit along slit lines422, 424 and 426 to form bump foil tab 414′ secured to bump foil sheet404′ along line 428. The tab may then be shaped and formed by stampingto form bump foil tab 414′. The embodiment shown in FIG. 24 in which aplurality of bump foil pads 414 and pads 416 may be formed from, andremain attached to, a larger foil sheet (404 and 406 respectively) maybe preferred for fabrication of small bearings, for example thosebearings with outer diameters of between 0.1 inch and 2 inches. It willbe appreciated that as the bearing size decreases the dimensions of pads414 and 416 will likewise decrease and may pose increasing challenges inreliably positioning and attaching these pads to a thrust plate 402.Larger bearings, say up to 100 inches in diameter, may optionally employindividual bump foils and pads or may continue to employ the foil sheetconstruction elements of the small bearings.

It may be noted that because the bearing may include a number of equallyspaced bearing elements 414, 416 the bearing may be readily split, forexample along c-c (FIG. 24) to facilitate installation. As with thesplit journal bearing it may be preferred to introduce features such asguide pins fitting into reamed holes, or shoulder screws engagingcomplementary partially-threaded holes (not shown) to assure alignmenton re-attaching the split bearing segments. To facilitate reassembly, itmay be preferred to split thrust plate 402 but to only partially splitbump foil sheet 404 and top foil sheet 406, for example from their outercircumference to centerline 408. For such a thrust bearing the sheets404, 406 may be elastically flexed to enable sufficient clearance arounda shaft, for example, toolholder portion 149 in FIG. 11, for bearinginstallation and removal.

Another embodiment of a bump foil configuration 414 is shown in planview in FIG. 26. As with the journal bearing, the bump foil may includea series of alternating flats 430 and ridges 432 here separated intoindividual sectors by circumferential slits 434. The heights of theridges 430 may vary with their location in the foil. In this figure theridge to ridge or flat to flat spacing ‘S’ is constant but otherconfigurations in which the dimension ‘S’ will vary with position on thefoil are also comprehended in this description. The relative ridgeheight may be indicated by the numbers 1-5 associated with each ridgewhere 5 may represent a large ridge height and 1 a small ridge height.Like the bump foil shown in FIG. 8, the foil of FIG. 20 may becircumferentially slit. The slits 434 may minimize ‘cross-talk’ betweenadjacent ridge-flat tabs and thereby enable each ridge-flat tab torespond to an applied load more independently of its neighbors. Slits434 may also facilitate cooling gas flow through the bearing.

Thus each ridge 430 may comprise only a portion of the overall radialdistance spanned by the overall bump foil 414. The ridge 430 heights mayvary systematically with position and the ridge height variationgenerally conforms to a compound wedge. The compound wedge may taperupward, both circumferentially from the leading edge of the foil to thetrailing edge, and also from the leading edge of the bump foil outercircumference to the trailing edge of the inner circumference. FIG. 27is a section taken along line 27-27 in FIGS. 18 and 20 and shows therelative ridge 430 height variation from the leading edge of the bumpfoil outer circumference to the trailing edge of the innercircumference. As will be appreciated by consideration of items 1-5, theheight of the bumps may gradually increase from item 1 to item 5.

As a consequence of this configuration, a cross flow may be induced inthe fluid in the composite diverging wedge region. In this region thefluid may be subject to a circumferential pressure gradient which mayencourage the fluid film to move along circumferential stream lines.However, there may also be centrifugal forces promoting radial flow.

These two components of the flow velocities may be orthogonal to eachother in the compound tapered region. This flow behavior, depicted as aseries of streamlines overlaid on the outline of a bearing pad segment414, may be shown at FIG. 28. The runner velocity may be in thedirection of arrow 440. The circumferential flow (stream lines 450)entering the bearing gap space may interact with radial,centrifugally-induced flow (streamlines 460, shown as dotted) and may bevery effective in transferring momentum to the radial flow and impartinga circumferential component to the stream lines. Thus little sideleakage (streamlines 462) may occur and most of the flow may exit thepad at the trailing edge flowing in a generally circumferentialdirection (streamlines 470). It will be obvious by reference to FIG. 24that this circumferential flow shown exiting pad 414 may become anincoming circumferential flow for the next, downstream, pad.

This flow behavior may have consequences for bearing cooling. Theradially-flowing air, drawn from the bearing inner circumference will becool. Some portion of the circumferentially-directed air may be cool airdrawn from beyond the outer bearing circumference as indicated at 452but a significant proportion will be previously-heated air drawn fromthe upstream pad. Since little side leakage 460 occurs, the small volumeof heated air which may be lost to side leakage may make littlecontribution to bearing cooling.

In FIG. 29 the effect of providing a pressurized airflow (streamlines480) at the outer circumference of the pad according to one embodimentis illustrated. Obviously the cooling airflow, acting in opposition tothe centrifugally-directed fluid (streamlines 460) may promote ingressof a much larger volume of cooling air (streamlines 450) anddramatically expand the region where an incoming flow of cool air isdominant, providing enhanced cooling. This, may result from the factthat the inwardly directed flow of cool air, in continuing to act on theexiting air flow, may direct a significant portion of the flow exitingat pad trailing edge inward, that is, toward the axis of rotation. Theinward flow may remove the heated air and permit expanded access ofcooling air as described above. Thus incoming streamlines 470′ from theupstream pad may be redirected toward the axis of rotation (408 in FIG.24) as shown figuratively by streamlines 472.

If the bearing is mounted on the outer diameter of the thrust plate itmay be fully accessible to an inwardly-directed radial airflow. Howeverif the bearing is mounted interior to the outer diameter it may bebeneficial to introduce openings or channels into the thrust plate toenable air access. The openings may be aligned with the gaps betweenpads. Such a configuration is illustrated in FIG. 30 which shows analternate bearing design 500 in which the top foil pads 512 and bumpfoil pads 514 are individually attached to the thrust plate 506. The topfoils shown in FIG. 30 have tabs 520 which may be secured, for exampleby welding to thrust plate 506; edges 521, 522 and 523 are unsecured.Note that this bearing may also be split, for example along line C′-C′,and that if split (not shown) the individual elements of the bearing mayincorporate features constructed and arranged for reassembly andalignment as described previously (not shown).

It is well known that a turbulent boundary layer is better able tomaintain its attachment to a surface than a laminar boundary layer whenthe fluid film flow velocities are transonic or supersonic so that aturbulent boundary layer may result in less pressure drag and less heatgeneration. For at least this reason it may be preferred to develop aturbulent boundary layer on the top foil.

Smooth, or flat, surfaces may promote a laminar boundary layer whileuneven surfaces or those of irregular height may be more likely topromote development of the more desirable turbulent boundary layer.Surfaces with generally uniformly-spaced height irregularities ofsimilar scale in regular spaced-apart configuration, for example dimpledsurfaces, may be especially effective in promoting turbulent boundarylayer formation across the entire surface.

Textured surface patterns with concavity and convexity, analogous todimples may be fabricated on the top foil and/or runner, or on coatingsapplied to them, using various techniques (laser beam, EDM, chemicaletching, etc.). The depths of such recesses may be about a fraction offluid film thickness (0.00002 to 0.0004 inch). However, such finefeatures may be worn away by any rubbing of the runner on the top foilwhich may occur during start-up and shut-down.

The wear process which may occur on start-up and shut-down may be usedto advantage since the wear may promote the development of the desiredspaced-apart surface irregularities. Consider the configuration shown inFIG. 26. Before a supporting air film is fully developed, runner 410 mayrub against top foil pad 416 (FIG. 24). But, top foil pad 416 may besupported by the corrugated annular segments 414 of bump foil sheet 404.Hence, the stiffness, and thus the local pressure during wear of the topfoil, may be greatest in those locations where the bump foil ridges 432(FIG. 27) directly support the top foil. This may produce non-uniformand local wear of the top foil creating an array of worn regionsdistributed across the top foil. These worn regions may form a series ofdepressions in the top foil during normal bearing operation and maypromote formation of the desirable turbulent boundary layer.

Wear of the top foil may be minimized by the addition of lubricious,wear-resistant coatings such as Korolon™ 1350, a proprietary,spray-gun-applied nickel-chrome coating with solid lubricants. But, themore effective the wear-resistant coating the greater the likelihoodthat the top foil may not acquire these wear-induced surface featuresresulting in a greater the tendency for the boundary air flow to remainlaminar.

In another embodiment, an alternative bump foil geometry 416′ may beused, as shown in FIG. 32 in which each of the series of the flats 430′and ridges 432′ in each of the tabs 421′, 423′, 425′, 427′, 429′ isoffset from ‘from the flat and ridges of its neighbor. Under the appliedpressure generated during use, such as shown in FIG. 16, those portionsof the top foil overlying the flats may be displaced downward more thanthe surrounding regions overlying the ridges to form a dimpled surfaceon the top foil. Since this dimpled surface may develop due to thepositional variation in stiffness of the top foil-bump foil combinationit may develop in the absence of top foil wear and similarly promoteboundary layer transition from laminar to turbulent.

In FIG. 32 the ridges and flats on adjacent tabs are shown as 180° outof phase. But lesser degrees of phase mis-match may be employed.Obviously the dimple pattern will be modified in the event that otherthan 180° phase mismatch is selected but, as described above, even at 0°phase mis-match the desired dimpling may develop due to wear.

A similar concept may be employed for journal bearings. In oneembodiment, a journal bearing may have fewer than 9 ridges per tab. If,for a specific bearing and ridge spacing the number of ridges wouldexceed nine, then multiple bump foils, each with fewer than 9 ridges, insufficient number to fully cover the bearing surface, may be employed.Preferably the ridges may be oriented perpendicular to the axis of shaftrotation but, ridges inclined at up to ±45° to the shaft rotation axiswill yield acceptable results.

In FIG. 30, foil thrust bearing 500 comprising thrust plate 506 withthrough holes 502 and 504 for attachment or alignment may have throughholes 508 for ingress of cooling air to a plurality of bearing pads 510comprising top foil pads 512 and bump foil pads 514. In this embodimenttop foil pads 512 and bump foil pads 514 may be attached to thrust plate506 only along line 520 and edges 521, 522 and 523 of top foil pad 512may be unattached. A similar procedure may be adopted for bump foil pad514. The attachment of the bump foil is not shown in FIG. 30 but bumpfoil pad 414 shown in FIG. 26 or bump foil pad 416′ shown in FIG. 32 maybe adapted for such procedure where flats 436 (FIG. 26) or 436′ (FIG.32) may be employed to weld the bump foil pad to thrust plate 506 (FIG.30).

In addition to the configurations and procedures identified forattachment of the bump foil pad to the thrust plate the trapezoidal orΩ-shaped retaining groove employed to retain the foil in the journalbearing may be adapted to attach the bump foil pad the a thrust plate.An example of such a bump foil pad is shown as 514′ in FIG. 25. TheΩ-shaped retaining feature 535 is evident and only minor changes to theslit configuration, here shown as two sets of circumferential slits 534and 534′ are necessary to accommodate this attachment means.

It has been noted that foil bearings, either journal or thrust may onlygenerate their own supporting air film only after the shaft surface hasattained some suitable and subsequent to rotation speed. Hence prior toexceeding that suitable speed on start up or subsequent to falling belowthat speed on shut-down, the shaft and top foil may be in loadedcontact. Thus wear of both the shaft and top foil may occur. Theoccurrence of wear may be reduced by appropriately coating the surfacesof the shaft and at least the shaft-contacting surface of the top foil.It has been found that an effective combination may be a hard,wear-resistant coating applied to the shaft and a soft, lubriciouscoating applied to the top foil. It has also been found beneficial toretain the wear debris, mainly contributed by the softer lubriciouscoating, within the foil bearing since even though detached from thesurface they continue to contribute lubricity to the foil surface.Debris retention may be well promoted by the top foil geometry whichleads to the ‘elasto-pressure’ dam shown at 209 in FIG. 12 which may bepromoted by the top foil-bump foil configurations herein described.

The above description of select examples of embodiments of the inventionis merely illustrative in nature and, thus, variations or variantsthereof are not to be regarded as a departure from the spirit and scopeof the invention.

1. A micro-machine constructed and arranged for high speed operationcomprising; a drive system; a housing assembled from at least twocomplementary parts defining at least a joint plane therebetween; aspindle having a rotation axis and constructed and arranged for supportby at least one gas-cooled foil journal bearing and at least onegas-cooled foil thrust bearing; the foil journal bearing and the foilthrust bearing being supported by the housing; the housing having gasflow passages arranged and constructed for conveying pressurized gas tothe foil thrust and journal bearings.
 2. The micro-machine as set forthin claim 1, wherein the drive system comprises at least one of agas-powered turbine and an electric motor.
 3. The micro-machine as setforth in claim 1, wherein the joint plane is substantially perpendicularto the spindle rotation axis.
 4. The micro-machine as set forth in claim1, wherein the joint plane is substantially parallel to the spindlerotation axis.
 5. The micro-machine as set forth in claim 1, wherein thespindle comprises a hollow cylindrical portion with an external diameterwhich terminates in a planar endcap from which outwardly extends a solidcylinder, smaller in diameter than the external diameter of the hollowcylinder, the solid cylinder comprising a tool holder.
 6. Themicro-machine as set forth in claim 5, wherein the spindle endcapcomprises at least one through hole inclined to the axis of rotation ofthe spindle.
 7. The micro-machine as set forth in claim 1, wherein thefoil journal bearing comprises a plurality of top foils overlying a likenumber of bump foils supported by a hollow, generally cylindricalhousing with an interior circumference; the top foils each having alength, a width and two ends, the ends being unsecured, each top foilhaving a mounting feature, extending across its width, for engagementwith one of a plurality of features of complementary shape in thehousing; the number of complementarily-shaped features in the housingbeing equal to the number of top foils; the complementarily-shapedfeatures being uniformly distributed around the interior circumferenceof the housing; the mounting feature of each top foil being locatedbetween the ends of the top foil to divide the length of the top foilinto a leading segment and a trailing segment; the lengths of the topfoils being constructed to ensure that a portion of the leading segmentof a first top foil positioned in a first complementary feature overlapsthe trailing segment of a second top foil positioned in a secondcomplementary feature immediately adjacent to the first complementaryfeature.
 8. (canceled)
 9. The micro-machine as set forth in claim 7,wherein the foil journal bearing is split into at least two portionsalong a cylindrical axis of the housing.
 10. The micro-machine as setforth in claim 7, wherein the gas-cooled foil journal bearing is cooledby flow of gas directed along the cylindrical axis of the housing. 11.(canceled)
 12. The micro-machine as set forth in claim 1, wherein thegas-cooled foil thrust bearing comprises a thin, disc-shaped thrustplate with a center and an edge, a plurality of bump foils and aplurality of top foils; each of the plurality of top foils overlying oneof the plurality of bump foils the top foils being generally positionedon the circumference of a circle centered on the thrust plate centre ingenerally uniform spaced-apart configuration, wherein the gas-cooledfoil thrust bearing is cooled by radially-inwardly directed flow of gasfrom the thrust plate edge to its center.
 13. A machine constructed andarranged for high speed operation comprising; a drive system; a housing;a spindle having a rotation axis and constructed and arranged forsupport by at least one gas-cooled foil journal bearing and at least onegas-cooled foil thrust bearing; the foil journal bearing and the foilthrust bearing being supported by the housing; the housing having gasflow passages arranged and constructed for conveying pressurized gas tothe foil thrust and journal bearings; the drive system, spindle andbearings being constructed and arranged to operate at speeds greaterthan 700,000 rpm.
 14. The machine as set forth in claim 13, wherein thedrive system comprises at least one of a gas-powered turbine and anelectric motor. 15-63. (canceled)
 64. The micro-machine as set forth inclaim 1, wherein the spindle is adapted for retaining a shank of acutting tool.
 65. The machine as set forth in claim 13, wherein thespindle is adapted for retaining a shank of a cutting tool.
 66. Amachine for machining microscopic features, the machine comprising; ahousing; a spindle which is rotatable within the housing and is adaptedfor retaining a shank of a cutting tool; means for rotating said spindleat speeds greater than 700,000 rpm, wherein said rotating means includesat least one foil journal bearing and at least one foil thrust bearingeach supported by said housing for supporting said spindle and whereinsaid rotating means further includes gas flow passages arranged in saidhousing for conveying pressurized gas to said foil thrust and journalbearings.
 67. The machine as set forth in claim 66, wherein the machineis assembled from at least two complementary parts defining at least ajoint plane therebetween.
 68. The machine as set forth in claim 66,wherein said means for rotating comprises an electric motor.
 69. Themachine as set forth in claim 66, wherein the drive system comprises agas-powered turbine, and the gas-flow passages are further arranged andconstructed to route exhaust gas from the gas-powered turbine to thefoil thrust and journal bearings.
 70. The micro-machine as set forth inclaim 66, wherein the spindle has a cylindrical cavity sized forshrink-fit retention of a shank of a cutting tool.
 71. The machine asset forth in claim 66, wherein the foil thrust bearing comprises: aplurality of generally planar top foil segments positioned so as to becircumferentially spaced relative to a rotating runner for engagement bythe runner when the bearing is mounted to bearingly support rotation ofthe runner; a plurality of bump foil segments underlying and supportingand engaging said top foil segments respectively; means including athrust plate for supporting and engaging said bump foil segments; Meansfor securing each of said top foil segments and said bump foil segmentswherein each of said top foil segments is secured only along an edgeportion which is trailing relative to the runner rotation leaving anedge which is leading relative to runner rotation and a pair of sideedges free of said securing means and wherein each of said bump foilsegments is secured only along an edge portion which is trailingrelative to the runner rotation leaving an edge which is leadingrelative to runner rotation and a pair of side edges free of saidsecuring means; each of said bump foil segments including a plurality oftabs separated by slots extending from said leading edge at leastpartially to said secured edge portion; and each of said bump foil tabscomprising a plurality of substantially parallel ridges separated byflats.